Interpretive Summary: Predicting minimal damage to beneficial plants is one of the most important steps in developing biological controls. Prior research indicated that the biological control agent, the Melaleuca snout weevil could feed and complete development on two other Australian bottlebrush plants from the same plant family grown as ornamentals in south Florida. Additionally, all larvae could not complete development to the adult stage when fed a native unrelated species wax myrtle. If larvae were transferred from their normal host, melaleuca during the third instar, they fed and completed development but with 75% mortality. These results were confirmed here in this study. Moreover, behavioral cues used by this insect to find and begin feeding could be found in all these species. Chemical similarity between the weed, melaleuca, the bottlebrushes, and wax myrtle explained why feeding occurred by larvae when presented with each plant. However, several unique compounds in wax myrtle that were not found in the normal host melaleuca or the bottlebrushes, explained the high mortality only when fed wax myrtle. These results indicate that when compiling a list of beneficial plants to test to determine suitability of potential agents, chemical similarity needs to be emphasized instead of only taxonomic similarity.

Technical Abstract:
Host range expansion in insect herbivores is often thought to be mediated by several factors; principal among them is secondary plant chemistry. In weed biological control, the host range of a prospective new agent is one of the most important considerations in the development of control agents. A host testing process seeks to determine the behavioral acceptance and nutritional value of different test plant species to the potential agent. A list of test plants is constructed that comprises species that are close taxonomic relatives of the target weed plus other species of economic or ecologic importance. The host testing of the Melaleuca quinquenervia biological control agent Oxyops vitiosa indicated that larvae would accept and complete development on the Australian target weed Melaleuca quinquenervia, two Australian ornamental species, Callistemon citrina, C. viminalis (all Myrtaceae). However, the larvae did not complete development when fed a North American species Myrica cerifera (Myricaceae). The study reported here confirms these results and examines the nutritional and performance differences in O. vitiosa larvae fed leaves of these species. The leaf quality factors, percent moisture, percent nitrogen, toughness, and terpenoid content were related to larval survival, performance and digestive indices. The results indicate that plant quality among the Mytaceae species was generally similar and correspondingly larval survival, performance and digestive indices differed little when larvae were fed leaves of these species. However, significant differences occurred in the plant quality of the North American M. cerifera compared with the Australian species which had leaves with the lowest percent moisture, lowest leaf toughness, highest percent nitrogen. This species, however, is not a nutritional host as none of the neonate larvae survived to pupate. When third instars were switched to M. cerifera from their normal host M. quinquenervia reductions were found in survival, biomass gain, digestive efficiency, and conversion of digested food to insect biomass. The marginal acceptance of this North American native plant in laboratory bioassays appears related to the terpenoid chemistry that has similarities to the taxonomically unrelated host M. quinquenervia. However, the high larval mortality corresponds to several novel terpenoids that are not present in the host. For weed biological control host testing these results indicate that M. cerifera is a poor laboratory host for O. vitiosa. Additionally, future test plant lists should include plants with secondary chemistry similar to the target weed as these compounds may constitute behavioral cues that are relevant to these specialized herbivores.